1. Field of the Invention
The present invention relates to a process for recovering acid and metal salts from spent liquors withdrawn from metal pickling baths, and particularly to a process for recycling the acid from the pickling liquors after removing the metal salts.
2. Background Art
Pickling liquors from the primary metal and metal finishing industries comprise a major source of toxic industrial wastes. Pickling is a method used in sheet and wire mills or metal fabricating plants to remove oxide and scale from metal sheet, strip, wire, or parts before another operation, such as galvanizing, electroplating, or painting, by passing the metal products through an acid bath. The spent pickling baths contain residual acid and metal salts of the pickling acid. A significant amount of these wastes comes from pickling baths in the iron and steel industries where the principal acids used are hydrochloric and sulfuric and the resulting metal salts are ferrous chloride and ferrous sulfate, respectively. In a hot-dip galvanizing operation, the pickling bath often is used as well to strip zinc from off-specification products. In this case, the pickling liquor also contains zinc salts of the pickling acid.
In a typical pickling operation for iron or steel, the pickling tank is filled initially with virgin sulfuric acid and water and is maintained at about 150.degree. F. During pickling, the acid reacts with iron oxide scale on the metal normally to form ferrous sulfate heptahydrate (FeSO.sub.4.7H.sub.2 O). As the operation progresses, the acid strength diminishes (from an initial value of about 12% by weight), and there is a gradual buildup of ferrous sulfate in the tank. Slowdown of pickling rate resulting from acid depletion can be delayed by replenishing the solution with virgin acid as required.
When the iron concentration reaches about 8% by weight, however, ferrous sulfate starts to crystallize in the tank and on the work pieces. In most facilities, therefore, the operation is suspended when the iron concentration reaches 8% by weight and the acid concentration falls to about 7% by weight. The solution in the tank is then drained and replaced by virgin acid and water. Usually the waste acid solution is hauled away for treatment to neutralize the acidity and precipitate the iron as ferrous hydroxide before disposal.
A few facilities have recovered the waste acid by passing the depleted solution through a refrigerated tank to crystallize FeSO.sub.4.7H.sub.2 O at temperatures near 32.degree. F. and then have returned the mother liquor to the pickling tank. As shown in FIG. 1, a pickle bath tank 11 receives metal work, denoted by arrow 12, in the form of sheet, strip, wire, or fabricated parts. After being descaled in the bath, the work leaves the tank (arrow 13). When the metal salts produced by the descaling reaction build up to a preselected value, a waste acid solution of metal acid salts is transferred via line 14 to a crystallizer tank 15. Brine or other refrigerant that has been chilled in a refrigeration unit 16 is delivered through line 17 to circulate through heat exchanger coil 18 in the crystallizer and then return to the refrigeration unit through line 19.
A portion of the metal salts crystallizes out of the waste acid solution as its temperature drops. The mixture of crystals and acid mother liquor passes through line 20 to a crystal dewatering unit 21, such as a centrifuge. The mother liquor constituting most of the initial waste acid, but depleted of a substantial amount of metal salts, returns as recovered acid via line 22 to the pickle bath 11, while the crystals are discharged from the dewatering unit via line 23. The heptahydrated ferrous sulfate crystals obtained in this way have a texture and particle size similar to table sugar. The crystals dewater very easily and make a commercially desirable by-product of the pickling operation.
Although acid recovery by refrigerated crystallization eliminates the need to neutralize and dispose of the waste acid and yields a useful by-product, it has at least two disadvantages. The recycled acid has essentially the same reduced concentration as the waste acid solution (about 10% by weight in the case of sulfuric acid pickling); so significant amounts of makeup fresh acid must be added to the pickling tank to bring the acid concentration to its original level. At the same time, some spent acid remains with the ferrous sulfate crystals (about 2-10% by weight of the crystals) so expensive acid-resistant materials must be used for shipping containers and for equipment used for further processing of these crystals.
A recent development is the use of diffusion dialysis to separate the acid from the metal salts in a waste pickling solution. As shown in FIG. 2 and described in a product brochure published by Pure Cycle Environmental Technologies, Inc. of Palmer, Mass. and disclosed in U.S. Pat. No. 5,264,123 issued to Daniel Bailey on Nov. 23, 1993, waste acid contaminated with metal salts from a pickle bath 11 passes through a line 24, pump 25, line 26, filter 27, and line 28 to an acid holding tank 29 in a diffusion dialysis system 30. Acid is metered from the holding tank to flow in contact with one side of ionic exchange membranes in a membrane stack 31. Water delivered through a line 32, filter 33, and line 34 to a fresh water holding tank 35 is metered in a counter-current fashion on the recovery side of the membranes in membrane stack 31. The majority of the acid migrates through the membranes into the water, leaving contaminants such as metal salts behind. The purified acid returns through line 36 to the pickling tank, while the contaminant-laden spent acid stream goes through line 37 to metal recovery or waste treatment for further processing.
An advantage of this process over refrigerated crystallization is that the acid can be recovered at a concentration of up to 95% of the strength of the waste acid; so when it is recycled to the pickling tank, very little virgin acid makeup is needed. A countervailing disadvantage, however, is that a stream of very dilute acid solution containing metal contaminants is discharged for treatment or recovery.